The operation of nuclear reactor power plants produces substantial quantities of low level radioactive wastes. For disposal, these wastes must be solidified.
The main sources of these wastes are:
A. Spent ion-exchange resins used to maintain an extremely high degree of purity in the water used in the BWR (Boiling Water Reactor). These resins are in the form of small beads and are delivered for solidification wet with about an equal weight of water.
B. Dilute sodium sulfate solution, contaminated with some radioactive nuclides, which is the result of the ion-exchange resin regeneration process.
C. Powdered ion-exchange resins, called Powdex, are coated onto a filter and used as an ion-exchange bed. The contaminated Powdex is delivered wet with water for solidification.
D. Filter pre-coats, such as diatomaceous earth, Cellulite and Solka-floc, become contaminated and are also delivered water-wet for solidification.
E. Boric acid solution recirculates through the PWR (Pressurized Water Reactor) and contaminated boric acid solution is removed for solidification and burial.
F. Cleanup solutions from floor scrubbings and from decontamination of equipment. These contain detergents, oxalic acid, phosphoric acid, potassium permanganate, potassium hydroxide and sodium hydroxide.
In current technology the solutions are concentrated in evaporators. The sodium sulfate can be brought to 20% solids and the boric acid to 12% solids in conventional evaporators. Any attempt to go to higher solids concentration results in serious scaling and corrosion. With a forced circulation titanium-tubed evaporator it is sometimes feasible to take the sodium sulfate to 25% solids. The evaporator bottoms, water-wet resins and filter-aids are mixed with portland cement or urea-formaldehyde (U-F) for solidification. This increases the volume by about 1.6 times. Much of the cement or U-F resin is used to solidify the water.
The cost of burying these solidified wastes currently is about $25/ft.sup.3. If the water could be removed before solidification, significant savings could be achieved.
The sodium sulfate forms the largest portion of the radioactive waste and provides a good example of the economics involved. Ten cubic feet of 20% sodium sulfate solution forms 16 cu ft of solidified radwaste when it is mixed with cement or U-F resin.
The 10 cu ft of 20% sodium sulfate solution contains 135 lbs of dry sodium sulfate. The bulk density of powdered sodium sulfate is approximately 100 lb/cu ft. When mixed with 35% of a binder the volume increases only 10% as most of the binder fills the interstices. Consequently, the 135 lbs of dry sodium sulfate, when mixed with 35% binder has a volume of 1.5 cu ft, slightly better than a 10:1 volume reduction when compared to U-F or cement solidification.
Several methods to reduce volume are being practiced today. One example is to calcine the materials to form solid granules. A second is to mix the materials into hot asphalt. All of these systems have their advantages and disadvantages but to date there has been developed no system which can solidify these low level nuclear wastes in a simple, low-cost, low-volume manner. According to this invention, however, such a system has been provided.
It is accordingly an object of this invention to provide a system for the continuous drying and the coating of the dried product.
Another object of this invention is to provide a coated and castable mixture having a low leach rate.
Another object of this invention is to provide an evaporating system which produces no scale.